Patent GB-1430138-A (GB)
Title: Image transmission systems
IPC: H04N1/024
Filed: 1973-06-01
Abstract: 1430138 Copying by scanning RICOH KK 1 June 1973 [7 June 1972] 26191/73 Heading H4F In a half-tone facsimile transmission system producing output signals characterizing at least four discrete grey levels, the attenuation or gain of the video signal obtained by line by line scanning is varied cyclically in discrete steps from line to line, e.g. such that the amplitude of the resultant video signal produced during each of a pair of odd lines, Fig. 4 (d) and (f), is greater than that of the corresponding video signal produced during the intermediate even line, Fig. 4(e), and the resultant video signal is applied to a threshold circuit (38), Fig. 2 (not shown), whose effective threshold level is switched cyclically during each line scan between at least two different levels V1 and V2 at a switching frequency f1, Fig. 4(b), so that as shown in Fig. 4 (g), (h), and (i), during a portion D of the video signal the output from the threshold circuit will be at a constant lower level L1; during a portion C of the video signal said output will be at said level L1 during each even line and will alternate at the switching frequency f1 between levels L1 and L2 during each odd line; during a portion B of the video signal said output will be at level L2 during each odd line and will alternate between L1 and L2 during each even line and during each portion A of the video signal said output will always be at level L2. As described the output from said threshold circuit is used to control an output gate responsive to pulses, Fig. 4 (a), of frequency f2 greater than f1, e.g. f2 = 2f1, so that the three portions C, B, A of the video signal result in corresponding differently characterized output pulse trains, the portion D being characterized by the absence of any output pulses. The cyclic variation of the effective threshold level of the threshold circuit may be effected by changing the actual threshold level of the threshold circuit or by using a threshold circuit with a fixed threshold level and cyclically varying (at frequency f1) the attenuation, Fig. 6 (not shown), or gain of the video signal applied to the threshold circuit. When a mechanical optical scanner is used, a line-to-line variation of the attenuation of the video signal produced by the associated electro-optical transducer may be effected optically and Fig. 1 shows such an arrangement in which each cycle of line-toline variation comprises two lines; in a modification, Fig. 7 (not shown), each said cycle comprises four lines. As shown in Fig. 1 a document 13 is moved over a transverse linear array of fibre optic bundles 121 ... 12n, an elongated lamp 11 being positioned above or below the document 13. The light emerges from the fibre optics along an arc of a circle which is scanned by a rotating assembly of two groups of fibre optic bundles 14, 15, optically coupled to a stationary photoelectric transducer 20, radially slitted masks 17 and 19 being attached to the input ends of the bundles 14, 15 and an attenuating filter 18 being provided to produce the line-to-line variation in attenuation. The assembly 14, 15 is rotated by a motor 24 which also rotates a disc 25 having three sets of slits 261 ... 26m, 271 ... 27l, and 281, 282 which in co-operation with three fixed slits 30 and associated photo-detectors 32, 33, 35 respectively produce: (1) a train of pulses of frequency f1 from which is derived the f2 pulses, Fig. 4 (a), applied to the output gate; (2) pulses, Fig. 4 (b), of frequency f1 for switching the effective threshold level; and (3) line frequency pulses. Fig. 5 (not shown) illustrates the image reproduced by the Fig. 1 system when scanning a document as in Fig. 3 (not shown).